To start up a large water jet propulsion unit for a ship, there is a need to have sufficient water in the impeller housing and/or take special measures, to obtain propulsion. This problem does not exist in conjunction with most water jet crafts, but only for ships powered by water jet, where of some reason a substantial portion of the impeller housing at occasion will be positioned above the water line, i.e. the water jet unit is not wholly submerged but is only partly located under the surface. Then it is necessary to carry out what is commonly known as “priming”, meaning that the impeller housing has to be filled with water, in connection with the start up phase, as is known per se.
In JP 7215294, a water jet propulsion unit is primed be means of vacuum from a tank connected to a vacuum pump, and further U.S. Pat. No. 3,970,027 discloses a priming means for a bow steering pump and utilizing a vacuum pump to fill the bow steering pump with water. Accordingly an additional pump has to be used which is costly and is also a risk factor from a view point of reliability.
JP 1262289 discloses a water jet propulsion unit, where quick starting is enabled by spraying a partial water jet pressurized by a pump impeller to the inside of a water duct of a front flow part of the pump impeller at the time of low headway by a water nozzle, and avoiding any cavitation at the pump suction side. Hence, also their solution uses additional machining causing the same disadvantage as mentioned above.
U.S. Pat. No. 5,634,831 discloses another known solution, that is complex and/or costly and that also includes aspects of uncertainty regarding reliability. It depicts a water jet propulsion unit using two counter rotating impellers. The nozzle section includes a throttled outlet to allow for a high mass/low pressure operation while maintaining pump priming. In one embodiment, the throttling device utilizes two spring-loaded flaps mounted inside the nozzle section upstream the ejection opening thereof and moving back into a recess provided in the wall of the nozzle section as the flow rate increases. In another embodiment, the throttling device includes a series of thin flexible strips fixed to a circular rim. A flexible rubber ring or a coil spring is provided at the free ends of the flexible strips to make the form a contracted nozzle opening. A thin rubber sleeve is fitted over the strips to prevent water loss when pressure increases and makes the nozzle opening expand.
JP-06-001288 shows a further known solution to assist in priming. Here a movable cone-shaped part is provided that is intended to be moved to a blocking position during priming, i.e. totally blocking the outlet. It is evident that such a solution is complex and costly. Moreover it requires complex control mechanisms, that are disadvantageous, not at least from the view point of reliability.
Further U.S. Pat. No. 6,422,904 B1 and WO 9821090, present known alternatives for enabling priming of a water jet propulsion unit. Both relate to small vessles using two counter rotating impellers and a spring-loaded flexible skirt, which helps to facilitate priming and control of pressure inside the unit. Also these latter solutions present disadvantages, and especially so in relation to larger water jet units.